1. Field of the Invention
The present invention relates to a method and an apparatus for optically reading a target on which optically readable information, such as an information code, for example, a barcode or a two-dimensional code, is written.
2. Description of the Related Art
Handheld optical information readers aim at reading an information code that is optically readable, such as a barcode, a two-dimensional code, or other similar codes. In this specification, a target itself or a target to which optically readable information is attached is collectively referred to as “target”.
For improving the handheld optical information readers, a type of handheld optical information readers that can read an information code positioned at a distance therefrom has been provided.
Such type of handheld optical information readers has a handheld body case provided at its one end portion with a reading window, a photodetector, such as a CCD (Charge-Coupled Device) area sensor, an imaging unit with an imaging lens, and a reading unit composed of a light illuminating device. The photodetector, the imaging unit, and the reading unit are installed in the body case, respectively.
In this type optical information reader, a user pushes a trigger switch attached to the handheld body case with the reading window of the handheld body case directed to a target, such as a barcode. The push of the trigger switch causes the light illuminating device to supply light through the reading window toward the target. The light supplied from the light illuminating device is irradiated to the target (barcode) so that light reflected from the target based on the irradiated light enters through the reading window into the imaging unit The light entered into the imaging unit is focused on the photodetector by the imaging lens to be imaged thereon, so that an image corresponding to the target is picked up by the photodetector.
In this type optical information reader, a certain range of a reading distance defined as a distance between the reading window and the target is previously determined depending on the relationship between an intensity of the irradiated light on the target and an exposure time of the photodetector. The determined range of the reading distance allows the reading apparatus to precisely read the target.
If the reading distance between the reading window and the target exceeds the determined range, the intensity of the irradiated light on the target would be insufficient, causing the need for repeat reading operations to arise.
For avoiding such repeat reading operations, conventional optical information readers each having means for measuring a reading distance between the reading window and a target in a noncontact manner have been well known.
First and second examples of the conventional optical information readers are disclosed in U.S. Pat. No. 5,426,288 corresponding to Japanese Unexamined Patent Publications No. H5-181995 and No. H6-162249.
The publication, as the first example, shows a barcode reader with a range sensor for measuring a distance between the reader and a target based on triangulation techniques. The barcode reader controls the diameter of a laser beam as an irradiated light on the target so that the diameter of the laser beam is appropriately set according to the target.
The publication, as the second embodiment, shows an optical information reader with means for measuring a period from which a laser beam is irradiated from the optical information reader toward a target to which a laser beam reflected from the target based on the irradiated laser beam is returned to the optical information reader. The optical information reader determines whether a reading distance between the reader and the target is within a predetermined range, which allows the optical information reader to precisely read the target, based on the measured period.
In the first example of the conventional optical information readers, the range sensor may be comparatively costly and may have a comparatively large size.
In the second example of the conventional optical information readers, because the velocity of the laser beam may be very fast, it may be difficult to precisely measure the period between the time of irradiation of the laser beam and the time of reception of the reflected laser beam.